Field of the Invention
The present invention relates to solar collectors, and more particularly to solar thermal collectors designed to heat a working fluid to temperatures within a range of 25° C. to 180° C.
Description of Related Art
The flat plate solar collector has not fundamentally changed since the 1970s. U.S. Pat. No. 4,011,856 to Gallagher (1977) teaches an exemplary design that fairly represents the vast majority of glazed flat plate solar collectors in use today. Such solar collectors typically employ tempered glass with reduced iron oxide content as the cover plate material. The glass must be thick enough to prevent sagging of unsupported spans. The glass must also have impact resistance and, in high wind zones, must be attached to the solar collector assembly with a frame and fasteners capable of withstanding the dynamic air pressures generated by severe windstorms. Consequently, the glass cover plate is typically either 3.2 or 4 millimeters thick, with corresponding weights of about 8 and 10 kg/m2. The use of glass makes flat plate solar collectors difficult to handle and expensive to ship. Also, tempered glass is not impervious to breakage. While tempering produces excellent flat surface impact resistance, the edge strength is poor. A sheet of tempered glass can shatter when lateral compressive force (for example, the force from a solar collector being dropped on its side during handling) drives the head or length of an adjacent glazing frame screw into the edge of the glass.
Translucent plastics, including acrylics and polycarbonates, have been used as solar collector glazing in an attempt to reduce weight and cost. Unfortunately, these materials suffer significant reductions in transmittance over time due to discoloration and structural degradation, caused by accumulated exposure to ultraviolet radiation and stagnation temperatures. Plastic glazing materials also tend to experience unacceptable levels of outward bowing when exposed to the dynamic pressures associated with severe windstorms.
An additional problem for both transparent and translucent cover plate materials is that airborne dust and grime can accumulate on the glazing surface, which reduces solar energy transmittance. In many climates, the frequency and intensity of rainfall are not sufficient to remove accumulated dust and grime. Further, it is a practical reality that periodic manual glass cleaning is problematic for solar collectors on residential rooftops and in large commercial arrays.
The side walls of a typical glazed flat plate solar collector are formed of four elongated, straight aluminum extrusions, with 45-degree beveled ends and various extruded appendages extending outward from the vertical wall portions. The four side walls are joined at 90-degree angle corners, often reinforced with L-shaped brackets on the interior side of each corner. Screws, bolts, or rivets tighten the side walls to the L-shaped reinforcing brackets. These sharp angle corner joints have two shortcomings. First, the solar collector side walls expand and contract with daily temperature variations. Over time, repeated expansion and contraction can lead to small gaps at the corner joints. This problem may be exacerbated by a difference in the upper horizontal side wall and the lower horizontal side wall temperatures, leading to small but meaningful differences in the longitudinal expansion and contraction of the upper and lower horizontal side walls. With even the smallest corner gaps, moisture and particulate laden air infiltration into the solar collector interior will inevitably occur, urged by pressure differences between the air inside the solar collector and ambient air. Moisture and particulate intrusion eventually end up as a grimy film on the underside of the glazing, reducing solar energy transmittance. The corner joints discussed are seldom, if ever, sealed with an elastomeric material that might help prevent moisture intrusion over time. The lack of effective corner sealing is due in part to the practical difficulty of sealing the abrupt, planar and pointed surface intersections at the top and bottom of each corner joint. While the corner joints could be welded, this strategy imposes unacceptable labor costs and is incompatible with the most common frame material and finish, which comprises an anodized finish of an aluminum frame.
During severe windstorms, square corners on a solar collector increase dynamic pressure on the cover plate. Comparative tests of roof gravel scouring showed that aerodynamic corners can double the damage threshold wind speed compared to conventional square corners. Pressure measurements showed up to 75% reduction in uplift pressures in a roof corner test for an aerodynamic corner, when compared with a square corner. See Lin, et al., “Aerodynamic Devices for Mitigation of Wind Damage Risk,” 4th International Conference on Advances in Wind and Structures. AWAS 08. Jeju, Korea, May 29-31, 2008.
A problem closely related to the wind uplift pressure issue discussed above is that any structure comprising a blunt windward edge with square corners promotes formation and shedding of vortex currents along the edges disposed downwind from such corners. See Okamoto, S. and Uemura, N. “Effect of rounding side-corners on aerodynamic forces and turbulent wake of a cube placed on a ground plane,” Experiments in Fluids, 11, 58-64. Springer-Verlag. 1991.) If the frequency of the vortex shedding happens to match the resonance frequency of the structure, the structure will begin to resonate and the structure's movement can become self-sustaining Vortex shedding on a solar collector perimeter during a severe windstorm can literally shake roof shingles loose and start progressive undermining of the entire roof structure, leading to eventual catastrophic loss. Roof deck failure is the leading cause of catastrophic residential building damage during severe windstorms. Once a building loses one or more pieces of roof deck, damage increases exponentially as vast amounts of wind-driven water enter the structure. Insurance claim data show that damage escalates quickly once a roof deck starts to fail. Even if the walls remain intact and the roof trusses do not fail, loss of the roof deck typically results in losses greater than 50% of building insured value. See Applied Research Associates, Inc. “Development of Loss Relativities for Wind Resistive Features of Residential Structures,” Florida Department of Community Affairs (DCA Contract 02-RC-11-14-00-22-003), Version 2.2, Mar. 28, 2002.
The evacuated tube solar collector is an alternative to the glazed flat plate solar collector. U.S. Pat. No. 4,067,315 to Fehlner and Ortabasi (1978) teaches an exemplary evacuated tube solar collector. The evacuated tubes of such solar collectors typically comprise 1.6 millimeter thickness borosilicate glass cylinders. While such glass cylinders do have a degree of impact resistance, breakage can nevertheless occur as a result of impacts by hail, wind-driven storm debris, errant golf balls and baseballs, and dropped tools. Breakage can also occur during shipping and handling.
Snow accumulation is also a problem for evacuated tube collectors. The vacuum that eliminates convective heat losses from inside the glass tube to ambient air also allows snow to accumulate on the glass tubes. The glass cover plate of a flat plate solar collector is warmed during sunny conditions by continual heat transfer from the hotter air on the underside of the glass to the cooler ambient air. However, the glass of an evacuated tube solar collector is only warmed by the small percentage of incident solar energy absorbed by the glass, and this heat is quickly lost to the cold ambient air that accompanies a snowfall. Thus, absent manual snow removal, evacuated tube solar collectors are rendered useless for some period of time after a snowfall.
Evacuated flat plate solar collectors have been proposed. U.S. Pat. No. 4,332,241 to Dalstein, et al. (1982), U.S. Pat. No. 7,810,491 to Benvenuti (2010) and U.S. Pat. No. 8,161,965 to Palmieri (2012) are exemplary designs. The Benvenuti '491 patent provides an excellent discussion of both tube and flat plate evacuated solar collectors. The Dalstein '241 patent teaches a complex double-walled frame, including an inner frame wall comprising four pieces of square tubing welded at 45-degree beveled sharp corners. The Dalstein '241, Benvenuti '491 and Palmieri '965 patents all teach various approaches to soldering or fusing metal to glass. These complex and relatively expensive processes are conceived to address the dissimilar materials and dissimilar rates of expansion and contraction of the glass cover plate and the metal frame walls. However, the Dalstein '241, Benvenuti '491 and Palmieri '965 patents do not solve the problems associated with flat plate solar collector glazing or sharp side wall corners, or the problem of snow accumulation on the outer glass surface of an evacuated solar collector.
The unglazed solar collector is another alternative to the glazed flat plate solar collector. Unglazed plastic solar collectors, with carbon black added to the plastic resin to enhance solar energy absorptance and combat the effects of prolonged exposure to ultraviolet radiation, are widely used for swimming pool heating. U.S. Pat. No. 3,934,323 to Ford, et. al. (1976) and U.S. Pat. No. 4,060,070 to Harter (1977) teach examples of unglazed plastic solar collectors. While swimming pool heating requires temperatures between 25° C. and 32° C., unglazed plastic solar collectors have been employed for potable water heating, which requires temperatures between about 45° C. and 60° C.
Unglazed solar collectors are capable of delivering 60° C. water when the flow rate of the working fluid is reduced. Unfortunately, though, unglazed solar collectors suffer much greater convective heat losses than glazed flat plate solar collectors when the ambient air temperature falls below the solar collector fluid inlet temperature. Wind compounds the problem. Poor cold weather performance is acceptable for solar swimming pool heating because the goal in most climates is only to extend the swimming season by a few extra months. Other water heating applications require hot water on a year-round basis.